In an effort to improve the efficiency of gas fired furnaces, induced draft blower types of furnaces utilizing a two-stage gas valve and an induced draft blower have been utilized. This type of furnace matches its heat output more closely to the heating demand of the residence being heated, and as such is more efficient than the older types of gravity draft-type furnaces.
In an effort to match the heat supplied from the furnace to the demand of the residence, a two-stage gas valve is operated at two different flow rates, and an induced draft blower is operated at two different speeds. This type of system has been operated with a control system that senses many parameters including the pressure within the combination chamber, the induced draft blower speed, and a signal to the gas valve to control it through its two-stage operation. This type of system is controlled by a microcomputer or microprocessor and is known as an integrated control system. These systems have been more efficient than the older types of furnace control systems, but certain safety considerations are required of the induced draft systems that are not significant in the older gravity draft-type systems.
These safety considerations are such as whether the gas valve in fact is functioning properly or is leaking, whether the induced draft blower is in fact operating at its proper speeds, and further whether or not the stack has become blocked due to some type of obstruction.